What's wrong with Audacity?
As far as I'm concerned, this is the only correct answer. Audacity is the best software for audio editing.
I deleted the comment because I'm not sure it's available as an Android app though.
WavePad is open source but it's free for non-commercial use and it's pretty nice. Handles a lot of formats out of the box.
*Edit: Oh, that would be for desktop. Just noticed I was in the Android /c
If you are comfortable with ffmpeg you can give termux a try. I used ffmpeg through it in the past to occasionally snip out parts from mp4's and that worked like a charm.
If you only want to trim MP3 or AAC files I can recommend mp3DirectCut. Ad-free, fast and simple.
Audacity used to be open source, several forks of it exist but easy to just download the last open source version of audacity (most Linux package managers will use this by default).
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